The present invention relates to an optical device using optical nonreciprocity and polarization rotation with use of an optical waveguide of a magneto-optical material. First, an optical isolator will be described as a typical example of an optical device using optical nonreciprocity.
An optical isolator is an element having a function of allowing light to pass only in one direction and blocking light propagating in an opposite direction. When an optical isolator is provided on an emission end of a semiconductor laser, light emitted from the laser passes through the optical isolator, so that the emitted light can be used as a light source for optical fiber communication. Light that is to enter into the semiconductor laser through the optical isolator is blocked by the optical isolator, so that it cannot enter into the semiconductor laser. If an optical reflection feedback enters into the semiconductor laser, the lasing characteristics of the semiconductor laser are deteriorated. Thus, the optical isolator blocks an optical reflection feedback, so that the semiconductor laser can maintain stable oscillation without deterioration of its characteristics.
If light enters in a direction opposite to an intended direction not only in a semiconductor laser, but also in a photoactive element such as an optical amplifier, then operational characteristics of the element are deteriorated. Since an optical isolator allows light to pass only in one direction, it can prevent light from entering into a photoactive element in a direction opposite to an intended direction. Optical isolators have widely been used for optical communication and the like.
All optical isolators currently being in practical use have no function of confining light within a cross-section perpendicular to a light propagation direction at a region through which the light passes. That is, they have no structure of waveguide effects. Those optical isolators are referred to as bulk optical isolators. It is difficult to integrate a bulk optical isolator with other waveguide elements such as a semiconductor laser. A bulk optical isolator is formed by integration of bulk elements including a polarizer, a nonreciprocal element of a magneto-optical material, a magnet for generating a magnetic field to control the magnetization direction of the magneto-optical material, and an analyzer.
Meanwhile, for integration with other optical elements, there have been proposed some optical isolators having waveguide effects, i.e., waveguide optical isolators. Those waveguide optical isolators use a waveguide of a magnetic garnet that has epitaxially been grown on a garnet substrate such as gadolinium gallium garnet (GGG). In Patent Document 1 (Japanese laid-open patent publication No. 7-56040), an iron garnet containing La, Ga, and Y is grown on a GGG substrate by a liquid phase epitaxial growth (LPE) method. An index of refraction is controlled by controlling the composition of a core layer and a clad layer, so that a waveguide layer is formed. Furthermore, a ridge portion is formed by wet etching to form a waveguide. The magnetization direction of the core layer is controlled by application of a magnetic field, and the nonreciprocity is measured.
Similarly, in Non-Patent Document 1 (T. Shintaku, Appl. Phys. Lett. 73 (1998) 1946), a Ce-substituted YIG garnet film is epitaxially grown with (111) on a GGG substrate to which Ca, Mg, and Zr have been added by an RF sputtering method. A ridged waveguide is formed by reactive ion etching. In order to cause isolation due to nonreciprocity, a specific angle should be formed between the light waveguide direction and the magnetization direction. The magnetization direction is controlled by an external magnetic field of a magnet.
Thus, in order to use a waveguide of a magneto-optical material as an optical device, the magnetization direction should be controlled by an external magnetic field. In view of size reduction and cost reduction of an optical device, it is desired to control the magnetization direction with a small external magnetic field or preferably without any magnetic field. For this purpose, the magnetic anisotropy of the formed magneto-optical material should be controlled precisely. A magneto-optical material that has epitaxially been grown exhibits strong growth induced magnetic anisotropy. Patent Document 2 (Japanese laid-open patent publication No. 8-253395) discloses a technique of heat treatment under at least 1000° C. in order to reduce such magnetic anisotropy. However, heat treatment of at least 1000° C. is excessively high in temperature as a process temperature of an optical waveguide device.
Patent Document 3 (Japanese laid-open patent publication No. 10-221720), which provides a waveguide optical part having a switching function, discloses a waveguide of a magnetic garnet, which is a magneto-optical material. The magnetic garnet waveguide is in the form of a rectangular parallelepiped elongated along an optical path, and the magnetization direction has a tendency to become parallel to a light traveling direction because of its shape magnetic anisotropy. Therefore, no magnet is required to control the magnetization direction of a waveguide formed of a magneto-optical material in a polarization rotation portion. However, since the magnetic garnet waveguide is epitaxially grown on a substrate, it exhibits strong growth induced magnetic anisotropy. Accordingly, it is difficult to control the magnetization direction without any external magnetic field of a magnet only by using the shape magnetic anisotropy. Thus, reduction or long-term variations of a polarization rotation angle disadvantageously occur.
Future achievement of nanophotonic devices in which optics and electronics are integrated on one chip has been demanded as great innovation technology. This achievement requires a technique of forming an LSI, such as a CPU or a memory, and a photoactive element, such as an optical switch or a laser, on the same substrate. A hybrid-type optical nonreciprocal element has been proposed as an optical nonreciprocal element on a silicon substrate. An optical reciprocal element is formed by a nonreciprocal mode converter and a reciprocal mode converter. A magnetic garnet waveguide is used only for the nonreciprocal portion. A half-wave plate is inserted in the reciprocal portion. All parts individually produced (the polarizer, the nonreciprocal mode converter, and the reciprocal mode converter) are inserted in a silica-based waveguide produced on a silicon substrate and fixed by ultraviolet-curing resin, so that the optical nonreciprocal element is produced. Insertion of the parts causes insertion loss at each portion. Additionally, this technology is disadvantageous in that it is extremely difficult to position the nonreciprocal mode converter with respect to the silica-based waveguide (see Non-Patent Document 2: N. Sugimoto et al., J. Lightwave Tech. 14 (1996) 2537).
Furthermore, Patent Document 4 (Japanese laid-open patent publication No. 2004-240003) discloses a technique of joining a magneto-optical material by wafer bonding, as a technique of forming a magneto-optical element on different types of substrates. In Patent Document 4, an SOI substrate and a second clad layer of a magnetic garnet are joined together by wafer bonding. In the SOI substrate, three layers including a core layer of silicon crystal, a first clad layer of SiO2, which is an insulating material, and a holder for holding the core layer and the first clad layer are stacked while the first clad layer of SiO2 is located as an intermediate layer. There is disclosed a magneto-optical waveguide having a structure of magnetic garnet/silicon/silicon dioxide in which the second clad layer of a magnetic garnet is attached to a surface of the core layer of the SOI substrate. In this case, the magnetization direction of the bonded magnetic garnet should also be controlled by an external magnetic field. Additionally, since a monocrystalline magnetic garnet is joined, this technology is disadvantageous in that size reduction is difficult and that the cost increases.
For integration on a silicon substrate, there has been demanded a technique of depositing a magneto-optical material with high crystallinity on a silicon or quartz substrate. A method of forming a YIG garnet film on MgO and SiO2 by sputtering has been reported in Non-Patent Document 3 (S. Y. Sung et al., Appl. Phys. Lett. 87 (2005) 12111). An amorphous film is formed on a substrate and crystallized by annealing with an RTA method to form polycrystals. The crystallization requires a high annealing temperature of at least 750° C. Because there is a difference in coefficient of thermal expansion between the substrate and the YIG garnet, inverse magnetostriction effects due to thermal strain by the annealing cause magnetic anisotropy. Therefore, a magnet is required for magnetization control of the waveguide.
Meanwhile, aerosol deposition (AD method) using a room temperature impact consolidation phenomenon has been developed as new film formation technology for oxides. The AD method employs a collision adhesion phenomenon of ultra-fine particulate materials. There has been expected achievement of a higher deposition rate and a lower process temperature as compared to conventional thin film formation methods (Non-Patent Document 4: Jun Akedo et al., Jpn. J. Appl. Phys. 38 (1999) 5397). Furthermore, because film characteristics do not depend upon an underlayer, any substrate can be selected in the AD method. The technique disclosed in Patent Document 5 (Japanese laid-open patent publication No. 2001-3180) relates to a formation method using an AD method. Mechanical impact is applied to ultra-fine brittle particulate materials supplied onto a substrate, thereby pulverizing the materials. Thus, the ultra-fine brittle particulate materials are joined together. Alternatively, the ultra-fine brittle particulate materials are joined together, and the ultra-fine brittle particulate materials and the substrate are joined together. With this method, it is possible to achieve bonding of ultra-fine particles and form a film at a high density with high strength without heating.
The technique disclosed in Patent Document 6 (Japanese laid-open patent publication No. 2002-235181) relates to a structure formed by an AD method. The structure is formed of polycrystal having no crystal orientation. Use of this AD method has been examined for thin film formation of an electro-optical material having high transparency (Non-Patent Document 5: Masafumi Nakada et al., J. of Crys. Growth, 275 (2005) 1275). The document has clarified that transmission loss of an AD film, which is a fundamental characteristic of an optical element, is caused by the Rayleigh scattering of fine particles forming a molded body and fine particles of an unmolded body having a different index of refraction.
The technique disclosed in Patent Document 7 (Japanese laid-open patent publication No. 2005-181995) relates to an optical element produced by an AD method, an optical integrated device, an optical information transmission system, and a method of manufacturing the same. An optical element is formed by an impact consolidation phenomenon in which mechanical impact is applied to ultra-fine brittle particulate materials supplied onto a substrate so that the ultra-fine brittle particulate materials are pulverized and bonded to each other for thereby forming a molded body. Pores included in the optical element and indexes of refraction of different phases have the relationship with an average diameter d (nm) of a portion different from a primary portion of the molded body and a wavelength λ (nm) of light propagating through the molded body such that d6/λ4<4×10−5 nm2.
An isolator has been described as an example of a device using nonreciprocity. Other optical nonreciprocal devices include an optical circulator. An optical circulator is formed by a magneto-optical waveguide as with an optical isolator. Furthermore, a polarization direction of light propagating through a waveguide can be controlled by using the polarization rotation of a magneto-optical material. Therefore, it is possible to form an optical device such as a polarization equalizer.